Nicotinic cholinergic signaling uses the transmitter acetylcholine to activate ligand-gated ion channels that are cation-selective in mammals. This form of signaling is widespread in the nervous system, reaches peak levels during early postnatal life, and continues throughout adulthood. It contributes to a variety of behaviors including arousal and cognition, participates in a number of neurodegenerative disorders including Alzheimer's and Parkinson's diseases, and is responsible for nicotine addiction. Despite intensive effort, little is understood about the role of nicotinic signaling during development when it drives spontaneous waves of excitation across the nervous system, and little is understood about the nicotinic mechanisms that subsequently exert global effects across networks in the adult brain. This proposal tests two novel hypotheses fundamental to these issues. The first is that nicotinic signaling during development promotes the formation of glutamatergic synapses both on early postnatal and adultborn neurons (Aims I & II). Since glutamatergic pathways provide the principal form of excitation in brain, this effect of nicotinic input is likely to have lasting consequences for nervous system function. The second hypothesis is that nicotinic activity in the adult brain can acutely and reversibly alter GABAergic signaling such that it stops being inhibitory and transiently becomes excitatory (Aim III). This could exert far-reaching effects across networks radically altering output. Preliminary studies performed on the hippocampus strongly support these ideas. The two major nicotinic acetylcholine receptors, homopentameric 7-nAChRs and heteropentameric 2-containing nAChRs, appear to complement each other in promoting glutamate synapse formation. One appears to act in cell-autonomous fashion to drive postsynaptic spine formation while the other may act indirectly to recruit components required for synaptic function. Preliminary results also support the second hypothesis: low levels of nicotine experienced by tobacco users may be sufficient to transiently invert the chloride gradient in adult neurons, thereby rendering GABA temporarily depolarizing. This could dramatically change the excitability of networks housing those neurons. The hypotheses will be tested by pharmacological and genetic manipulation, including loss-of-function and rescue experiments, performed on hippocampal slices and in vivo. The underlying molecular mechanisms will be analyzed and their consequences evaluated for system function. Imaging and electrophysiological approaches will be combined in this analysis. The experiments proposed here test pivotal ideas about the purpose of nicotinic cholinergic signaling in the nervous system. The results are likely to change how we think about fundamental processes guiding development and regulation of function in neural networks. The vulnerability of these processes to exploitation by tobacco-derived nicotine gives this work compelling health-related significance.